Techniques are provided for measuring angles of rotation of a rotatable body, and, in particular, for measuring angles of rotation of a rotating member that can rotate greater than 360 degrees.
In the automotive industry, some applications use angular measurements of steering wheels. For example, power steering units, electronic stability control, rear wheel steering systems, headlight steering, and roll prevention systems can all utilize a measurement, of the angular position of the steering wheel. Since the angular position of the steering wheel can vary by more than 360 degrees, an angular rotation sensor that can sense more than 360 degrees of rotation is used. In most automotive applications, a multi-turn sensor capable of measuring six complete rotations of the steering wheel is sufficient to cover the entire range of motion of a steering wheel.
Some examples of systems that have been used to measure the rotation of a rotatable body in a range greater than 360 degrees include optical sensors that use multi-turn counters and specialized algorithms to combine a turn count code with measurements from one or more single turn optical sensors. Another technique that has been used is gear reduction ratio, using the Vernier concept, for example. However, when using gear reduction ratio, the resolution and accuracy of the sensors is adversely affected. For example, if the gear ratio of the main gear to the small gear is six to one, then the accuracy of the angular sensor will be reduced by a factor of six as well. In many cases, this may not provide enough accuracy for the systems using the angle of rotation measurement.
Therefore, it is desirable to have an improved multi-turn sensor that provides a high degree of accuracy for multiple turns of a rotatable body, such as a steering wheel column, while still being simple and easily programmed such that it can be built at low cost and in a small package.